A Design of Two-Degree-of-Freedom Digital Controller for Hydraulic Servo Systems. Continued Report. A Countermeasure for Deterioration of Stability Caused by a Saturation Characteristic.

Abstract

This paper is concerned with the application of a two-degree-of-freedom (2DOF) digital controller to a hydraulic servo system. For suppression of a disturbance effect, the controller has an integral term and this causes instabilities by interacting with a saturation characteristic included in the actual hydraulic plant. To avoid the instabilities, a supplementary feedback loop is added to the 2DOF control system so as to suppress the input signal into the integral term under saturation conditions. A stability analysis is made for the system with that feedback loop and a nonlinear saturation element. Experiments are conducted under various conditions and from the results it has been clarified that the 2DOF digital hydraulic servo system has the prescribed desired input and disturbance response characteristics and that, even under saturation conditions, there occur no instabilities if the gain of the supplementary feedback loop is determined in the absolutely stable region obtained analytically.